What is it?

Magdump said:

I’m sticking with my educated guess. I think it’s a repurposed 30-06 brass. As for what, I can’t say. I reckon there might be a rifle out there somewhere that would shoot it, lol, but I don’t think that case started life in its current state.

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Definitely agree with you, it's a reformed 30-06 case. But formed into what? I've reformed '06 into 7.7 Jap, 7.65 Mauser, and even 8mm Mauser. Lots of 270, 308, 30-06, and anything else with that size case head have been formed into 7.92x33 for a friend's MP-44.

dougstump said:

Wow, I love a puzzle! One more dimension that may help, the diameter at the shoulder. Unfortunately I am over at our weekend house in Texas and I neglected to bring the books I need with me. It might be Monday or Tuesday before I can put my hands on them.

You clearly have a GI 30-06 case that has been reformed into something. In the photo you can see where the shoulder used to be. There are several possibilities:
1. It's a military round.
2. It's a wildcat or some obscure proprietary cartridge.
3. Some "new" cartridge someone is working on.
4. It's a practical joke. One time I necked a few 357 mag cases down with a 30 Luger die & left at the public range. I even left one "live" cartridge.

I'll let you know if I make a positive ID.

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diameter at shoulder is .430 Ring on neck looks to be crimp grouve or channelure

Blue Diamond said:

diameter at shoulder is .430 Ring on neck looks to be crimp grouve or channelure

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Thanks! Unless I turn up something else, I'm gonna say it's a 7.7x58mm Arisaka for the Type 99 rifle. All the measurements are within the ball park except the boolit diameter, it should be a .311~.312 not a .308. I doubt they were getting very good accuracy with these boolits. Definitely reformed from a GI 30-06 case.

Signing off for a few days. Have a little room left in the freezer that needs to be filled with hog before my season closes on the 5th. I'll check back end of week to see if this has been figured out. I really thought this would have been solved by now with all the firearms experts on this forum. Cartridge seems to an original, not reloaded or reformed. Did U.S. armories make foreign rounds back in 54? Primer seems to original with crimp and sealer. Also cartridge seems to be ideal for filling the gap between /06 and .308

It could be reformed from USGI Brass that had bullets pulled. Every now and then i even see brass liked that on CMP Forum for sale.

Everything on the history of the plant says that it worked with Federal Cartridge company to produce 30 and 50 caliber munitions for US military and I actually saw one reference to .45 cal ammo having been loaded there at one time, which I’ve seen samples of. The plant was active for WWII, Korean Conflict and the Vietnam War, and is said to have been on standby during the years between. I see no mention of production for other military outside of the US, or any commercial loading. It was primarily run by the US Army and apparently was not a commercial production plant.
Whatever the case it’s been nice to do a little research on the plant’s history. For so many years collecting mil surp ammo, I never really thought much about it except that when it came to cheap lots of mil surp ammo, there was Lake City, WCC, and Twin Cities.

Once I got the dimensions, there's no mystery. It's simply a Twin Cities made .30-06 case that was reformed into a 7.7X58mm. I have reformed many cases, in the photo you can clearly see where the original shoulder and neck were. For some reason, whoever form the case used a .308 boolit instead of a .311.

I finally got caught up enough to dig out the 7.7 Jap dies and formed one from a 30-06 case. Here is a 30-06, the formed 7.7, an original 7.7 Jap round, and a GI 7.62. That's what you found. Again, I have no idea why it has a.308 boolit.